Sunday, July 30, 2017

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why is the sky blue and where does it start? – Oliver Scott, age 7, Wombarra.

This is something that parents get asked every day. And it’s a great question, Oliver!

Some people think the sky is blue because of sunlight reflected off the ocean and back into the sky. But the sky is blue even in the middle of the countryside, nowhere near the sea!

Others think it’s because of the water in our atmosphere. But the sky is blue in places that are extremely dry, like the desert.

A blue sky over the Sahara desert in Libya.Wikipedia

So what’s the real reason?

The sky is blue because of the way sunlight interacts with our atmosphere.

If you’ve ever played with a prism or seen a rainbow, then you know light is made up of different colours. The name “ROY G. BIV” helps us remember these colours: red, orange, yellow, green, blue, indigo, and violet.

A rainbow over my house in suburban Melbourne, 2017.Duane Hamacher

These colours make up just a tiny portion of the electromagnetic spectrum, which includes ultraviolet waves, microwaves, and radio waves. This means the invisible waves that cause sunburns, allow us to heat-up our leftovers, and let us listen to the radio are all forms of light.

The spectrum of light, showing the wavelength with objects of comparable size.NASA

Light moves as waves of different lengths: some are short, making bluer light, and some are long, making redder light. As sunlight reaches our atmosphere, molecules in the air scatter the bluer light but let the red light pass through. Scientists call this Rayleigh scattering.

The spectrum of light we can see. Each colour from red to blue looks has a shorter distance between the waves.NASA

When the Sun is high in the sky, it appears its true colour: white. At sunrise and sunset, we see a much redder sun. This is because the sunlight is passing through a thicker layer of our atmosphere. This scatters the blue and green light along the way, allowing the redder light to pass through and illuminate the clouds in a beautiful array of red, orange, and pink.

Rayleigh scattering can affect how we see the Moon. When the Moon passes through the shadow of the Earth during a total lunar eclipse, blue and green light is scattered in the Earth’s atmosphere, letting red light pass through. Our atmosphere acts a like a magnifying glass, refracting (bending) the red sunlight onto the Moon. This can give it an eerie dark red hue.

Please tell us your name, age and which city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! We won’t be able to answer every question but we will do our best.